动词的时态：since 的四种用法1） since +过去一个时间点（如具体的年、月、日期、钟点、1980 ， last month， half past six） 。例如： I have been here since 1989. 1989 起，我一直在这儿。2） since +一段时间+ ago.例如：I have been here since five months ago. 我在这儿，已经有五个月了。3） since +从句。例如：Great changes have taken place since you left. 你走后，变化可大了。Great changes have taken place since we were here. 我们走后，变化可大了。4） It is +一段时间+ since 从句。例如：It is two years since I became a postgraduate student. 我考上研究生有两年了。
